"Currently our assessment is that 14 million people are identified as in need of humanitarian assistance," by 2017
Boko Haram jihadists have laid waste to the impoverished region since taking up arms against the government in 2009, displacing millions and disrupting farming and trade.
Nigerian President Muhammadu Buhari has reclaimed territory from the Islamists but the insurgency has taken a brutal toll, with over 20,000 people dead, 2.6 million displaced from their homes, and famine taking root.
UN humanitarian coordinator Peter Lundberg said the crisis was unfolding at "high speed."
"Currently our assessment is that 14 million people are identified as in need of humanitarian assistance," by 2017, Lundberg told reporters in the Nigerian capital Abuja.
Out of them, 400,000 children are in critical need of assistance, while 75,000 could die "in (the) few months ahead of us," Lundberg said.
The UN hopes to target half of the 14 million people -- a population bigger than Belgium -- with the Nigerian government working to reach the rest.
But Lundberg said that the UN did not have enough money to avert the crisis and called on international partners, the private sector and Nigerian philanthropists to "join hands" to tackle the problem.
"We need to reach out to the private sector, to the philanthropists in Nigeria," Lundberg said.
"We will ask international partners to step in because we can only solve this situation if we actually join hands."
Maiduguri, the capital of northeast Borno state and birthplace of Boko Haram, has doubled in size to two million people as a result of people seeking refuge in camps for internally displaced people.
Despite the World Food Programme warning of "famine-like conditions", the UN has not declared a "level three" emergency, the classification for the most severe crisis that would draw more attention and desperately needed funds to Nigeria.
"The humanitarian response hasn't scaled up adequately to meet a growing demand for food, particularly in the more remote camps in the northeast," Roddy Barclay, intelligence analyst at consultancy firm Africa Practice.
Nigerian vigilante and security sources told AFP in September that at least 10 people were starving to death daily in a camp for IDPs near the Borno capital.
There is also the ongoing issue of insecurity. Despite the recent military gains, Boko Haram still prowls the northeast region and stages attacks and suicide bombings.
"The Nigerian army has scored notable military successes in containing Boko Haram. But that's not to say they have stabilised the region entirely," Barclay said.
"Movement in remote zones remains high risk and the focus remains overwhelmingly on furthering military gains rather than addressing the very real socio-economic impact of the crisis."
Those zones include the shared borders of Nigeria, Niger, Cameroon and Chad in the Lake Chad Basin, said Ryan Cummings, director at intelligence firm Signal Risk.
"The scale of the humanitarian disaster in northeast Nigeria has been grossly underestimated," Cummings said.
"There's an estimated one million people still living in communities inaccessible because of the ongoing insecurity."
Now the fear is that Boko Haram will try to capitalise on the failure of the Nigerian government -- and the international community -- to save the hungry.
"There are many claims that resources allocated to IDP camps are being misdirected into avenues of corruption, so aid is not reaching the people," Cummings said.
Boko Haram could prey on that anger, he said, warning that "they could potentially end up being recruited back to Boko Haram."